1. Field of the Invention
The present invention relates to a mist spraying apparatus and an image forming apparatus, and more particularly, to an apparatus which converts a liquid into a mist and sprays the mist by using an ultrasonic wave, and an image forming apparatus which records an image by means of a group of minute liquid droplets (mist cluster) sprayed as a mist.
2. Description of the Related Art
In the related art, Japanese Patent Application Publications No. 62-85948, No. 62-111757, No. 2-134250, No. 5-57891, No. 2002-59549 and No. 2002-166541 disclose ejection technology based on a mist system in which minute liquid droplets are ejected in a group (cluster) by using ultrasonic wave vibration. Furthermore, there is a non-patent reference, “Investigation into ink droplet ejection in print head using concentrated ultrasonic wave and nozzle”, (Shumpei Kameyama, et. al., Journal of the Acoustical Society of Japan, Vol. 60, No. 2, pp. 53-60, 2004) which relates to the basic structure and ejection mechanism disclosed in Japanese Patent Application Publications No. 2002-59549 and No. 2002-166541.
FIG. 9 is a plan diagram of a nozzle surface in a mist type head in the related art, and FIG. 10 is a cross-sectional diagram showing the composition of a liquid droplet ejection element corresponding to one nozzle (one channel). As shown in FIG. 9, a mist spray head 200 in the related art comprises a nozzle plate 212 having ejection openings (nozzle holes) 210. A diaphragm 214 and piezoelectric elements (vibrators) 216 are disposed to the rear of the nozzle plate 212 (the lower side in FIG. 10). The space between the diaphragm 214 and the nozzle plate 212 is filled with ink.
The piezoelectric elements (vibrators) 216 bonded to the diaphragm 214 each comprise a common electrode 218, a piezoelectric body 219 and an individual electrode 220. When a drive voltage is applied between the two electrodes, the piezoelectric element 216 vibrates and applies a planar wave from below toward a free surface (which is commonly called “meniscus”) of the liquid at a nozzle hole 210, thereby inducing a surface tension wave (capillary wave) due to the particular characteristics (surface friction, etc.) of a nozzle edge 222. Moreover, if the frequency of the planar wave and the onset amplitude at the meniscus satisfy prescribed conditions which is dependant on the properties of the liquid, then time series oscillation of the surface tension wave occurs. Consequently, at a certain time point, minute liquid droplets 226 break off from wave peaks of the surface tension wave 224. The topics described above describe the mechanism of creating a capillary mist.
However, in the mist method based on the related art, there are the problems described below.
At first, the ejection direction varies and the dot diameter expands, because of variations in the accuracy of the shape of the nozzle edge, Coulomb repulsive force between minute liquid droplets, and the like.
Secondly, there is a problem of variation in the size of the liquid droplets. Since minute droplets are ejected from the free surface of liquid inside the nozzle in accordance with the stochastic distribution of the surface energy, the droplet size depends on the stochastic distribution of the surface energy. Thus, according to the related art, it is difficult to achieve a uniform droplet size. Moreover, if a nozzle is an ideal circular nozzle having axial symmetry, the stochastic distribution of the surface energy has axial symmetry and the cluster of liquid droplets is theoretically ejected from the nozzle in the torus fashion. However, in the actual practice, it is inferred that the creation of a mist is due principally to the occurrence of an axial asymmetry of the stochastic distribution of the surface energy which is dependent on the probabilistic broken symmetry of the nozzle, and the like. Since the probabilistic broken symmetry of the nozzle is not an available parameter, then it is difficult to control the size of liquid droplets. In this way, in a mist system in the related art, there is a problem of variation in the size of the liquid droplets.
Thirdly, due to the combination of variation in the ejection direction and the liquid droplet size as described above, there is density non-uniformity in dots formed by a mist cluster which has been deposited on an ejection receiving medium.
Finally, the head has poor characteristics for removing air bubbles because of the sealed structure thereof.